1. Technical Field
The present invention relates to a liquid crystal display (LCD) device and in particular, an LCD device which controls a viewing angle.
2. Related Art
LCD devices, especially, LCD devices adopting thin film transistors (TFT) are widely used in various applications such as mobile phones, large-sized televisions, a personal display device, etc. A user may desire to keep other users from viewing a display screen of a personal display device (“a secret mode”). In other cases, a user may desire to share a display screen with a large number of users (“a wide view angle mode”).
FIG. 6 illustrates a related art LCD device 600 having a secret mode and a wide view angle mode. In FIG. 6, a backlight 630 emits light to the rear surface of a liquid crystal display panel 610. The backlight 630 has high directivity. Between the common liquid crystal display panel 610 and the backlight 630 having the high directivity, a liquid crystal display layer 620 is disposed. The LCD layer 620 operates to switch between a scattered state and an unscattered state. The LCD layer 620 is, for example, a polymer dispersed liquid crystal display layer (a scattering-unscattering switching layer).
When the LCD layer 620 is in an unscattered state, the light emitted from the backlight 630 proceeds only from rear to front, as shown in FIG. 6. If other users are positioned at the side of the liquid crystal display panel 610, the displayed image may not be viewable at the side. On the other hand, if the LCD layer 620 is in a scattered state, the light emitted from the backlight 630 proceeds to the side directions as well as the front direction, as shown in FIG. 6. The displayed image may be viewable at the side and shared by a large number of users. However, expenses for manufacturing the LCD device 600 may increase because of the LCD layer 620.
FIGS. 7-10 illustrate a vertical alignment type liquid crystal display device in the related art. A polarizer has its axis directed in the vertical direction, and an analyzer has its axis directed in the horizontal direction, as shown in FIGS. 7A and 7B. FIGS. 7A and 7B show the shape of liquid crystal molecule which is viewed from front. In FIG. 7A, no voltage is applied and the liquid crystal molecule is aligned vertically. As the birefringence of the liquid crystal molecule does not occur, no light is transmitted. When the voltage is applied to the liquid crystal display panel as shown in FIG. 7B, the liquid crystal molecule is inclined upward. The optical axis of the liquid crystal molecule is in parallel with the absorption axis of the polarizer. Also, double refraction of the liquid crystal molecule does not occur and any light is not transmitted.
FIG. 8A illustrates the shape of a liquid crystal molecule which is viewed from the side at an angle and FIG. 8B illustrates the shape of a liquid crystal molecule which is viewed from the front of a vertical alignment LCD device. In FIG. 8A, no voltage is applied and the axis of the liquid crystal molecule is in parallel with the absorption axis of the analyzer. No light is transmitted. On the other hand, the voltage is applied in FIG. 8B and the axis of the liquid crystal molecule is offset from the axis of the polarizer or the axis of the analyzer. Consequently, birefringence of the liquid crystal molecule occurs and light is transmitted.
Light leakage phenomenon may substantially lower display contrast in the horizontal direction. As a result, data may not be recognizable when the display is viewed in the horizontal direction. Confidentiality of the display may be obtained using this light leakage phenomenon. FIG. 9 illustrates specific construction for controlling confidentiality of the display. In FIG. 9, a single pixel includes sub-pixels of red, green and blue (RGB) and a sub-pixel of white (W). FIG. 10 illustrates an arrangement of liquid crystal molecules of the respective sub-pixels shown in FIG. 9. As shown in FIG. 10, the alignment of the liquid crystal molecules in the white sub-pixel is different from those of the liquid crystal molecules in the RGB sub-pixels. Specifically, the liquid crystal molecules are aligned upward and downward in the white sub-pixel. Consequently, when no voltage is applied to the white sub-pixel, the white sub-pixel does not contribute to the display. Accordingly, a normal display may be realized. When the voltage is applied to the white sub-pixel, the white display appears in the horizontal direction. The contrast of the display is lowered in the horizontal viewing angle alignment. Accordingly, it is difficult for other people to view the displayed image.
Another related art LCD device is a fringe field switching (FFS) mode LCD device, which includes a common electrode in shape of “<” to improve a viewing angle. FIG. 11 is a plan view illustrating each of RGB pixels for a related art FFS mode LCD device. FIG. 12 illustrates operations of liquid crystal molecules as a voltage is applied to the related art FFS mode LCD device. As shown in FIG. 11, the related art FFS mode LCD device includes a common electrode which is formed in shape of “<” to regulate the inclination direction of liquid crystal.
As shown in FIG. 12A, when no voltage is applied to the LCD device, the liquid crystal molecules are aligned in the vertical direction. If a voltage is applied to the LCD device, the liquid crystal molecules are inclined in the predetermined direction. The inclined direction is determined based on the effect of the inclined electric field applied to the common electrode, i.e., the direction perpendicular to the extending direction of the common electrode, as shown in FIG. 12B. As a result, the liquid crystal molecules are inclined in the two directions corresponding to the “<” shape. The LCD device may have a good viewing angle. However, the LCD devices may not obtain confidentiality of the display upon need. Therefore, there is a need of an LCD device that overcomes drawbacks of the related art LCD devices.